1. Field of the Invention
This invention relates to an ink ejecting device for recording an image which is designed to eject at least two types of ink, such as differently colored inks. In particular, this invention is directed to ensuring a constant drop volume or a constant dot size for drops or dots of the different types of ink.
2. Description of the Related Art
Traditional impact recording devices are now being replaced by non-impact type recording devices, and the market for non-impact type recording devices is expanding. One type of conventional non-impact type recording device, an ink ejecting recording device, has the simplest operating principles and is easily able to produce multi-scale and color images. Of the known types of ink ejecting recording devices, the drop-on-demand type ink ejecting recording device, which is capable of ejecting ink droplets only when required to form a dot of the image, has rapidly become very popular due to its good ejecting efficiency and its low running cost, among other beneficial features.
Known examples of such drop-on-demand type ink ejecting recording devices include a Kyser-type device disclosed in U.S. Pat. No. 3,946,398, a thermal ejecting type disclosed in U.S. Pat. No. 4,723,129, and a shear-mode type disclosed in U.S. Pat. Nos. 4,879,568, 4,887,100, and 5,016,028.
In forming an image by using any of these known types of ink ejecting devices, the volume of the ink droplets to be ejected changes as a shape and/or a size of the ink chambers change, as a shape and/or a size of the nozzles changes, as the physical properties of the ink changes, and/or as a magnitude and/or a pulse width of a driving voltage changes. Other factors can also affect the ink droplet volume. In forming a monochromatic image, the shape and the size of the ink chambers, the shape and/or the size of the nozzles, and the magnitude and the pulse width of the driving voltage are all strictly controlled across the array of nozzles so that there is no, or at most a minimum, variation in the volume of the ink droplets ejected from the array of nozzles. Accordingly, the diameters of the image dots formed by the ink droplets on a sheet of paper are essentially uniform, and a beautiful image is formed.
However, when forming a color image, a plurality of differently colored inks are used, each of which includes different components from the others. It is therefore difficult to provide differently colored inks having constant physical properties.
The volume of an ejected ink droplet is a function of the product of the velocity in an ejecting direction of a meniscus passing through a nozzle and the time required for the meniscus to pass through the nozzle in the ejecting direction. Accordingly, if a fixed driving voltage is applied to eject different ones of the inks, where each of the differently colored inks has different physical properties, such as different viscosities, the viscous resistance of each of the inks to flowing through the corresponding ink chamber is different, because of the different viscosities. This causes a difference in the velocity of the meniscus passing through the corresponding nozzle for each of the differently colored inks, resulting in the volume of the ink droplet for each of the differently colored inks being different.
To solve this problem, Japanese Unexamined Patent No. Publication No. 5-261941 discloses a method of adjusting the dot diameters by changing the magnitude of the driving voltage. In contrast, to solve this problem, Japanese Unexamined Patent No. Publication No. 6-155766 discloses a method of adjusting the dot diameters by changing the pulse width of the driving voltage. The result of each of these proposed solutions is to make the volume of the ink droplets for each of the differently colored inks to be equal to each other. However, these methods require either a plurality of power supplies or a complicated driving circuit, increasing the cost of the non-impact type recording device.
U.S. Pat. No. 4,380,771 discloses a method for making the ejection velocity of the ink droplets of the differently colored inks constant, by changing the magnitude of the driving voltage according to the different lengths of the ink chambers in an ink ejecting print head, wherein the lengths of the ink chambers are different according to which one of the differently colored inks is used in each ink chamber. However, in this method, a plurality of power supplies are also required to change the magnitude of the driving voltage. Thus, this method also requires a complicated driving circuit, increasing the cost of the non-impact recording device.
Furthermore, even if the volume of the ink droplets for each of the differently colored inks are made equal to each other, a difference in the surface tension of each of the differently colored inks causes a difference in the dot diameter of the ink dots formed on the paper for each of the differently colored inks. As a result, a beautiful color image cannot be created.